According to the Coriolis principle, there always acts on a moved mass, when in a system a rotational movement of the mass superimposes with a straight line movement of the mass directed at least partially perpendicularly to the rotational axis, an additional force referred to as the Coriolis force. This effect is utilized in known manner in Coriolis, flow measuring devices to determine, for example, a mass flow of a fluid flowing in a pipeline. Coriolis, flow measuring devices have, as a rule, one or more measuring tubes, wherein these, depending on device type, can be formed into different configurations. The system of the at least one measuring tube (and, in given cases, more add-on parts, such as counteroscillator arms, etc.) forms an oscillatory system, which, depending on measuring tube configuration, has corresponding natural oscillation modes, such as, for example, bending oscillations (fundamental mode as well as modes of higher order), torsional oscillations (fundamental mode as well as modes of higher order), etc.
For use, a Coriolis, flow measuring device is inserted into a fluid carrying pipeline in such a manner that the fluid flows through the at least one measuring tube. The fluid is, in such case, preferably formed by a liquid, which, depending on application, can have different viscosities and, in given cases, can entrain also solids and/or gas. For determining a mass flow of the fluid, the at least one measuring tube is excited by at least one exciter to execute oscillations. The at least one exciter can, in such case, be in the form of, for example, an electromechanical exciter, especially an electrodynamic exciter, which exerts on the measuring tube a force corresponding to a supplied exciter current. As a rule, the oscillatory system is excited to a resonant frequency of the same (for example, the fundamental mode of the bending oscillation). If fluid is not flowing through the at least one measuring tube, then the entire measuring tube oscillates in phase. If fluid is flowing through the at least one measuring tube, then a Coriolis force acts on the moved mass (of the fluid). This leads to the fact that the measuring tube is supplementally deformed due to the Coriolis force and a phase shift occurs along the direction of elongation of the respective measuring tube. The phase shift along a measuring tube can be registered by corresponding oscillation sensors, which can, in turn, be formed by electromechanical, especially electrodynamic, sensors and which are arranged spaced from one another along the direction of elongation of the measuring tube. The phase shift registrable via the oscillation sensors is proportional to the mass flow through the measuring tube.
Additionally or alternatively, Coriolis, flow measuring devices can also measure other physical variables, such as, for example, density or viscosity, of a fluid flowing in a pipeline. In the case of density measurement, the principle is utilized that the resonant frequency (for example, of the fundamental mode of the bending oscillation) depends on the oscillating mass and therewith on the density of the fluid flowing through the at least one measuring tube. By adjusting the excitation frequency in such a manner that the oscillatory system is excited in its resonant frequency, the resonant frequency and therefrom, in turn, the density of the flowing fluid can be determined.
In the case of mass flow measurement as well as also generally in the case of measuring a physical measured variable of a fluid flowing through a Coriolis, flow measuring device, the physical measured variable to be determined, such as, for example, mass flow, density, viscosity, etc., of the flowing fluid is calculated, in each case, from at least one registered variable, such as, for example, at least one sensor voltage, and, in given cases, additional variables. Included in these calculations are, among other things, device-specific factors, which are determined, for example, earlier in the context of a calibration. Such device-specific factors can, however, change over time. Especially, there occurs in the case of many applications of Coriolis, flow measuring devices, over time, abrasion (especially in the case of particles entrained in the fluid), corrosion (especially in the case of aggressive media) and/or coating (especially in the case of media, which tend to form accretions) of the at least one measuring tube. The thereby related changes of the oscillatory behavior of the at least one measuring tube bring about a measurement error in the case of measuring a physical measured variable of a flowing fluid, especially in the case of mass flow measurement.
Desirable, in such case, is that such abrasion, corrosion and/or coating of the at least one measuring tube can be detected, without it being necessary to remove the Coriolis, flow measuring device from service or otherwise interfere with its operation in some substantial way. Furthermore, it is desirable that, for performing the diagnosis, measuring (especially of mass flow, density and/or viscosity) is not interrupted. This means that such diagnosis can also be performed continuously or regularly at predetermined time intervals. Moreover, it is desirable that no or only small additional energy consumption be required for performing the diagnosis.
Known are different diagnostic methods, by which the usual parameters K0 and Q for description of the oscillation characteristics of a Coriolis, oscillatory system are determinable. In such case, K0 refers to the stiffness of the oscillatory system (especially of the at least one measuring tube and/or oscillatory arm, etc.), while Q is the quality factor. An example of a known diagnostic method is that of determining the quality factor Q from the resonant frequency fr, from the frequency f−45 for a phase shift of −45° and from the frequency f+45 for a phase shift of +45°, as in the following Equation (1):
                    Q        =                              f            r                                              f                              -                45                                      -                          f                              +                45                                                                        (        1        )            
This known diagnostic method requires, however, a considerable time period, since, in each case, one must wait until the oscillatory system has tuned to the relevant frequencies. Furthermore, as a rule, the flow measurement must be interrupted during the performing of the diagnostic method. Accordingly, there is in the case of known diagnostic methods a great need for improvement as regards the above named, desirable requirements.